1. Field of the Invention
The present invention is broadly concerned with methods and apparatus for the determination of temperatures of an object by utilizing magnetic field-induced eddy currents in a conducting member forming a part of or in operative thermal communication with the object. More particularly, the invention is concerned with remote, noncontact temperature determination methods and apparatus wherein characteristic time constants of the eddy currents are ascertained and used in calculating the temperature of the conducting member.
2. Description of the Prior Art
There is a strong demand in modern industry and other fields for remote, noncontract temperature sensing devices. This demand is not satisfied by known infrared thermometers, given that these require an unobstructed line of sight for operation. The most advanced RFID circuits with integrated temperature sensing elements tend to be expensive and require reliable information transfer conditions, (e.g., RF) which restricts their use.
It is well known that alternating magnetic fields produce electromotive forces that excite eddy currents in electrically conductive objects. These currents are in the form of closed vortices, with the shape and space distribution of these vortices being defined by the alternating magnetic field pattern in space and time, and by the conductivity and magnetic properties of the conductive objects. Such closed vortices are considered as closed contours with current flow characterized by certain inductance and resistance values.
Attempts have been made in the past to utilize the eddy current phenomenon in order to measure the temperatures of the conductive objects. These efforts have not been fully successful, however. U.S. Pat. No. 5,573,613 describes a method an apparatus for sensing a temperature of a metallic bond line (susceptor) in an inductive welding process employing a conductive susceptor at the interface between two plastic parts. A magnetic work coil generates an alternating magnetic field through the plastic parts and around the susceptor. This in turn heats the susceptor, and the electrical resistance thereof changes as a function of the thermal coefficient of resistance of the susceptor material. Such resistance changes are reflected back as a change in the magnetic coil impedance. An electrical circuit senses the varying resistances, and such changes are translated into sensed temperatures. The sensed temperatures may then be used to adjust the power to the magnetic work coil, or the speed of travel of the work coil along the bond line. This technique does not require line of sight for operation. However, a significant drawback of this method is the dependence upon work coil impedance changes, which varies significantly with the distance between the work coil and the susceptor. Thus, this distance must be carefully maintained to ensure temperature measurement accuracy.
U.S. Pat. No. 3,936,734 describes remote measurement of conductivities and/or temperatures of metal components by means of the eddy current effect induced within the metal component by an alternating magnetic field. This magnetic field is produced by an excitation coil driven with alternating current arranged so that its axis perpendicular to the surface of the metal component. Also, a pair of measuring coils of equal radius are arranged coaxially and symmetrically with respect to the excitation coil at each end of the latter. The two measuring coils are connected electrically in series, and the phase angle between the current in the measuring coils and the current in the excitation coil is taken as an indication of the measured variable. In order to reduce the effect of distance changes, the measuring coils are placed at such a distance from the metal component surface that the phase angle between the excitation coil signal and the measuring signal is maximized. This method is inconvenient in practical use, however, owing to the necessity of mechanically adjusting the distances from the metal component to the sensor coils for each measurement.
See also, JP2000193531A; Ueda et al, Development of Methodology for In-Service Measurement of Transient Responses of Process Instrument used in LMFBR, International Corporation and Technology Development Center; Takahira et al., Impedance Variation of a Solenoid Coil Facing a Moving Sheet Conductor, Electrical Engineering in Japan, Vol. 103, Issue 3, pp. 1-7 (1983); and Keller, A New Technique for Noncontact Temperature Measurement of Rotating Rolls, Iron Steel Eng., Vol. 57, No. 5, pp. 42-44 (May 1980).